DevTools: [Console] log new url after main frame gets navigated

The patch logs a new url into console after main frame is navigated.
User will see the message only in case of "preserve log upon navigation"
setting; otherwise, it gets wiped by the following "messagesCleared"
event.

BUG=406999
R=vsevik,pfeldman

Review URL: https://codereview.chromium.org/611813002

git-svn-id: svn://svn.chromium.org/blink/trunk@182846 bbb929c8-8fbe-4397-9dbb-9b2b20218538